Servo drive system and continuous working system of press machine

A servo drive system of a press machine, comprising:

• a ram;
• an operation shaft which vertically moves the ram;
• and
• a servo motor which operates as a power source of the ram, wherein the servo motor uses a torque to generate ram pressure, the torque being based on at least one speed-torque characteristic of the servo motor;
• wherein the operation shaft is directly driven using the servo motor;
• wherein the at least one speed-torque characteristic of the servo motor is set such that a speed curve and a torque curve of the at least one speed-torque characteristic rise in a normal rotation direction to constant values and a ram position curve of the ram starts lowering substantially uniformly from an upper end position of the ram;
• the ram being operated such that when a striker provided at the lower end of the ram pushes a punching die an a tip end of the punching die abuts against an upper surface of a workpiece and the striker receives a load from the workpiece, the torque curve abruptly rises and the speed curve is reduced, thereby the lowering motion of the ram position curve decreases;
• the ram being operated such that when the tip end of the punching die lowers to a position near to a lower surface of the workpiece and the load received from the workpiece is abruptly reduced, the torque curve abruptly lowers, and the speed curve is accelerated beyond the constant value to restore the speed reduction, thereby the lowering speed of the ram position curve is also accelerated;
• and
• the ram being operated such that in a second half of an one cycle stroke of the ram, the ram position curve substantially uniformly rises from the lower end position of the ram to the upper end position of the ram.